1. Field of the Invention
The present invention relates to a self-oscillation type switching power supply unit.
2. Description of the Related Art
Until now, ringing choke converters have been often used as self-oscillation type switching power supply units. FIG. 1 is a circuit diagram of a conventional ringing choke converter (hereinafter referred to as RCC). As shown in the drawing, a switching transistor Q1 is connected in series to a primary winding N1 of a transformer T, and a control circuit including a phototransistor PT as a light receiving element of a photo coupler is connected to a feedback winding NB of the transformer. Furthermore, a controlling transistor Q2 is connected between the gate and source of the switching transistor Q1.
A rectifying and smoothing circuit made of a rectifying diode D3 and a smoothing capacitor C5 is provided between the two terminals of a secondary winding N2 of the transformer T. Furthermore, a voltage divider circuit comprising resistors R9 and R10, and a voltage detector circuit comprising a shunt regulator SR, a light-emitting diode PD of the photo coupler, and a resistor R8 are provided in the output portion of the rectifying and smoothing circuit.
The operation of the circuit shown in FIG. 1 is as follows. First of all, at the start when the power is turned on, a voltage is applied to the gate of the switching transistor Q1 through a starting resistor R1, and the switching transistor Q1 is turned on. Thus, an input power supply voltage is applied to the primary winding N1 of the transformer T, and a voltage having the same polarity as that of the primary winding N is generated in the feedback winding NB. This voltage signal is supplied as a positive feedback signal to the gate of the switching transistor Q1 through a resistor R2 and a capacitor C2. Because of the voltage of the feedback winding NB, a charging current flows into a capacitor C3 through a diode D1, resistors R3 and R5, and a phototransistor PT of the photocoupler. When the charged voltage of the capacitor C3 exceeds the forward voltage between the base and emitter of the controlling transistor Q2, the controlling transistor Q2 is turned on. Because of this, the voltage between the gate and source of the switching transistor Q1 becomes substantially zero and the switching transistor Q1 is forced off. At this time, a forward-bias voltage to the rectifying diode D3 is generated in the secondary winding of the transformer T, and thus the energy stored in the transformer T while the switching transistor Q1 is turned on is discharged through the secondary winding N2. Furthermore, at this time, the capacitor C3 is reverse charged by the flyback voltage of the feedback winding NB through resistors R6 and R7 and a diode D2.
When the voltage of the capacitor C3 reaches the forward-bias voltage between the base and emitter of the controlling transistor Q2 or less, the controlling transistor Q2 is turned off. When the energy stored in the transformer T is discharged through the secondary winding and the current flowing the rectifying diode D3 reaches zero, the switching transistor Q1 is turned on again by the kickback voltage generated in the feedback winding NB. Then, the above operation is repeated.
Here, the output voltage on the load side is detected by the voltage divider circuit of the resistors R9 and R10, the detected voltage is applied as a controlling voltage to the shunt regulator SR, and the amount of current flowing in the light-emitting diode PD of the photocoupler is charged in accordance with the detected voltage. Thus, the amount of light received by the phototransistor PT as a light receiving element of the photocoupler changes, and, through the change of the impedance, the charging time-constant of the capacitor C3 is changed. The more the output voltage decreases, the larger the above charging time-constant becomes, and accordingly the more the output voltage decreases, the longer the period from the turn-on of the switching transistor Q1 to the turn-off of the switching transistor Q1 by the controlling transistor Q2, that is, the ON period of the switching transistor Q1 increases, and, as a result, the output voltage is increased. In this way, constant-voltage control is performed so that the output may be held constant.
In the conventional self-oscillation type switching power supply unit of an RCC operation mode as shown in FIG. 1, it is known that the oscillation frequency f of the switching transistor Q1 changes substantially in inverse proportion to the input power or output power. This can be shown by the relationship of oscillation frequency f to output power P0 as shown in FIG. 2.
Generally, the lighter the load becomes, the more the switching loss per unit switching time decreases, but, as in FIG. 2, the lower the output power P0, that is, the lighter the load, the higher the oscillation frequency becomes, and the higher the oscillation frequency f becomes, the more the number of occurrence of switching losses per unit time increases, and accordingly even if the load becomes lighter, the percentage of reduction in the switching losses is very little. Therefore, the lighter the load, the more the efficiency of the power supply unit decreases.
In order to reduce the switching losses at such light load, the circuit constants may be designed to decrease the oscillation frequency at the rated load, but, when it is required to cope with a very wide range from a light load to a heavy load, the oscillation frequency at light loading inevitably relatively increases. That is, generally, the oscillation frequency at the rated load is determined by factors such as the flux density of the transformer, ripple, noise, etc., and when the oscillation frequency is set to be too low, saturation of the transformer, etc., are caused.
In the past, to solve the above-described problems of the self-oscillation-type switching power supply unit, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-235036, the loss during waiting time is improved by inputting a switching signal during waiting time thus forcing the oscillation frequency to become lower. Furthermore, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-278945, the loss during waiting time is improved by continuously lowering the oscillation frequencies over the range from the frequency at the rated load to the frequency during waiting time. FIG. 3 is the frequency characteristic of the switching power supply unit disclosed in the above Japanese Unexamined Patent Application Publication No. 11-235036, and FIG. 4 is the frequency characteristic of the switching power supply unit in the above Japanese Unexamined Patent Application Publication No. 2000-278945.
However, each of the above switching power supply units has problems described below.
Case 1: Switching power supply unit in Japanese Unexamined Patent Application Publication No. 11-235036
In this switching power supply unit, the losses are improved during waiting time, but when switched to the normal operation mode, the RCC operation is performed. Because of this, the loss at light loading cannot be improved during the RCC operation, and accordingly the increased input power and the heat-production problem of the switching transistor cannot be solved. Alternatively, it is possible to provide for intermittent oscillation, and, in this case, a problem occurs in that the ripple in the output is increased. An example where the power supply unit becomes loaded lightly is the standby mode of a printer.
Case 2: Switching power supply unit in Japanese Unexamined Patent Application Publication No. 2000-278945
In this switching power supply unit, when the switching power supply unit is lightly loaded, the oscillation frequency automatically decreases, but, in this case, when the oscillation frequency is lowered too much, the response characteristics of the load become worse. Therefore, the decreasing frequency is required to be set higher than the above-described frequency during waiting time in case 1, and the improvement of the loss during waiting time is less than that in case 1.
It is an object of the present invention to prevent the degradation of response characteristics in the RCC operation and to greatly improve the efficiency during waiting time such that the oscillation frequency is controlled so as to maintain a constant oscillation frequency or slowly lower the oscillation frequency at light loading and such that the oscillation frequency can be substantially lowered by a switching signal during waiting time.
In order to solve the above-described problems, self-oscillation type switching power supply units according to the present invention are constructed as described below.
In a first aspect of the present invention, a self-oscillation type switching power supply unit comprises a transformer having a primary winding, a secondary winding, and a feedback winding; a switching transistor which self-oscillates by receiving a feedback signal from the feedback winding and which makes the current of the primary winding flow and stop; a rectifying and smoothing circuit connected to the secondary winding; and an oscillation frequency control circuit for extending an OFF period by preventing the switching transistor from turning on for a fixed period after the switching transistor has been turned off such that a control signal to be input to the switching transistor is controlled.
At the rated load exceeding a fixed current, the oscillation frequency control circuit is set in a first operation mode in which a normal ringing choke converter operation mode is performed to lower the oscillation frequency as the load becomes heavier; at light loading in which the load current is a fixed current or less, the oscillation frequency control circuit is set in a second operation mode in which the OFF period is extended so that the oscillation frequency may be constant or slowly decrease as the load current is reduced; and the oscillation frequency control circuit contains a switch and is set in a third operation mode in which, when the switch is in a fixed state, the oscillation frequency is lowered further than in the second operation mode.
In the present invention, at the rated load exceeding a fixed load current, the oscillation frequency control circuit is set in a normal operation mode (first operation mode), that is, in a normal RCC operation mode, and, at a light load, in which the load current is a fixed current or less, the oscillation frequency control circuit is set in a standby mode (second operation mode), in which the OFF period is extended such that the oscillation frequency becomes constant or slowly reduced as the load current decreases. Furthermore, by a switching signal of the switch, the oscillation frequency control circuit can be set in a wait mode (third operation mode) such that the oscillation frequency is more reduced than in the standby mode. In this way, the RCC operation is performed in a normal operation mode, the oscillation frequency is controlled to be constant or slowly decrease in a standby mode, and the oscillation frequency can be controlled to further decrease by the switch in a wait mode. Therefore, since the oscillation frequency is not decreased too much in a standby mode, the response characteristics of the load are prevented from worsening, and since the oscillation frequency is greatly decreased in the wait mode, the loss can be prevented from increasing. For example, when this switching power supply unit is applied to printers, it is possible to set the switching power supply unit in a normal operation mode when the printer does printing, in a standby mode when the printer is powered on and is ready for printing, and in a wait mode when the printer is powered off, and accordingly, while reliability is maintained, power consumption for the entire printer can be lowered.
In the present invention, the oscillation frequency control circuit comprises a capacitor which is charged while the switching transistor is turned on and is discharged while the switching transistor is turned off, a first controlling transistor for preventing the switching transistor from turning on until the voltage of the capacitor reaches a fixed voltage when the capacitor is discharged, and a second controlling transistor for keeping the first controlling transistor turned off while the switching transistor is turned on; and the switch keeps the fixed voltage lower in the fixed state.
When the switch is set in a fixed state (for example, in the ON state), the operation of the first controlling transistor for prohibiting the turn-on of the switching transistor is made longer. Thus, since the period in which the turn-on of the switching transistor is prohibited is made longer, the oscillation frequency is reduced. That is, in the wait mode (third operation mode), the oscillation frequency decreases.
In the present invention, the oscillation frequency control circuit comprises a capacitor which is charged while the switching transistor is turned on and is discharged while the switching transistor is turned off, a first controlling transistor for preventing the switching transistor from turning on until the voltage of the capacitor reaches a fixed voltage when the capacitor is discharged, a second controlling transistor for keeping the first controlling transistor turned off while the switching transistor is turned on, and a discharge circuit in which the capacitor is discharged by applying the charged voltage of the capacitor to the control terminal of the switching transistor; the switch being provided in the discharge circuit; and the amount of the discharged current is decreased in a fixed state.
Here, the discharge circuit in which the charged voltage of the first capacitor is applied to the control terminal of the switching transistor is provided, and the amount of this discharge current is made changeable. Also, in such a construction, when the switch is in a fixed state, that is, in a wait mode, the ON period of the first controlling transistor is made longer, and accordingly the oscillation frequency is decreased.
In the present invention, the switch is switched on and off by an external signal.
Preferably, the switching means is made to be switched on and off by an external signal. Therefore, when a normal operation mode (first operation mode), a standby mode (second operation mode), and a wait mode (third operation mode) can be detected in an external circuit, these modes are possible to be automatically set.
In the present invention, a load detector for detecting light and heavy loading, and a switching circuit for switching the switch are provided such that the oscillation frequency control circuit is set in a third operation mode after a fixed period when the load detector has detected light loading.
Here, even if the load changes from a normal operation mode (first operation mode) to a wait mode (third operation mode), the oscillation frequency control circuit is not immediately shifted to the wait mode (third operation mode), and the oscillation frequency control circuit is changed into the wait mode (third operation mode) in a fixed period. In this way, even if the load often quickly changes, since the oscillation frequency control circuit operates in the normal operation mode (first operation mode) for the period, the reliability does not deteriorate. On the other hand, the wait mode (third operation mode) can be immediately shifted to the normal operation mode (first operation mode).